(1) Field of the Invention
The present invention relates to a method and an apparatus for directly working a material to be worked, with irradiating plural ultrashort-pulse laser beams. Particularly, the invention relates to a laser-working method and an apparatus for forming a two-dimensionally or three-dimensionally fine shape in a short time. The invention is directed to a method and apparatus for producing MEMS elements requiring high-precision parts needing fine shapes and master plates thereof as well as optical elements such as diffraction optical elements, holographic elements and photonic crystals and master plates thereof.
(2) Related Art Statement
JP-A 10-113782 and JP-A 5-57464 describe methods for working target objects through continuously irradiating plural short-pulse laser beams. They are aimed at preventing damages of a vicinity around a working site through irradiation with intense laser beams having a short time width. Further, JP-A 2002-324768 and JP-A 11-221684 describe a method for working a target object by continuously irradiating plural ultrashort-pulse laser beams.
Ordinary laser working is effected by utilizing thermal effects, while the ultrashort-pulse laser working can be effected in a non-thermal manner. Thus, the latter is known as a working method capable of reducing damages and an attached matter in the vicinity of the working site.
In the working method for directly removing a portion of the target part with the short-pulse laser beams, the worked shape is controlled in such a manner that a pattern is drawn by moving an irradiated position of the laser beams or the target object or a pattern is projected through a mask. In such a case, since the method in which the laser beams or the target object is to be moved takes a long time for working. Thus, the mask pattern-projecting method is superior from the standpoint of forming the pattern in a short time. The mask pattern-projecting working method is usually adapted for forming two-dimensional shapes, and can also work three-dimensional shape by using a gray scale mask. Further, there is available a method for working complicated two-dimensional shapes or three-dimensional shapes through irradiating laser beams at plural times.
However, the mask pattern-projecting working method has the following demerits that (1) a highly precious mask is required for forming a complicated shape and production of such a mask is difficult and costly, that (2) there is low flexibility in terms of worked shapes, and that (3) it is difficult to produce a complicated three-dimensional shape.
On the other hand, the working method with use of the gray scale mask has the following demerits that (1) the production of the mask is more difficult and costly as compared with the ordinary masks and that (2) the worked shape is influenced by the characteristics of the gray scale mask itself.
In order to solve the above problems, a working method in which the laser beams are irradiated at plural times, while the irradiating shape or the intensity distribution of the laser beams is being varied, may be considered. For, when plural interfering laser beams are simultaneously irradiated while being overlapped in space, interfering fringes appear to deteriorate a worked state. Therefore, working needs to be successively performed by successively irradiating plural laser beams. According to this method, however, working is influenced at a second irradiating time and thereafter by reflection resulting from changes in shape of a worked surface, scattering resulting from changes in working efficiency and roughening of the worked surface, and absorption and scattering of laser beam due to further roughening of the worked surface and matters scattered by working, such changes and laser beam absorption and scattering occurring by a first laser beam irradiation. As a result, a desired shape cannot be finely formed on the working surface.
Under these circumstances, the present invention solves the problems of the above conventional laser workings, and is to provide an ultrashort-pulse laser working method which can produce various fine shapes, particularly various fine three-dimensional shapes as well as a working apparatus performing the above working method and optical elements (structural bodies) produced by the above working method and apparatus.
The present invention is directed to a fundamental construction of the laser-working method, which comprising:
splitting a single pulse laser beam having a time width of not more than 500 picoseconds into plural pulse laser beams;
subjecting said plural pulse laser beams to time delaying, and adjusting a working surface-irradiating shape, an intensity distribution and a working surface-irradiating position of each of the plural pulse laser beams;
working a target object to be irradiated, while the respective working surface-irradiating shapes of said plural laser beams are partially or entirely spatially overlapped with each other or one another; and
setting to a time interval of from a pulse width to 1 nanosecond a time period during which each of the pulse laser beams is irradiated.
In the direct working with the ultrashort pulse laser, the surface shape of the target object begins to change around a few or several nano seconds after beginning the irradiation of the laser beam. In view of this phenomenon, before the surface shape begins to be changed with a first laser beam all the remaining pulse beams are continuously and successively applied to the working surface of the target object. Consequently, the object can be worked reasonably with the plural laser beams without being influenced by the changes in the surface shape, particularly edges of worked portions. Further, since the plural laser beams are successively irradiated upon the working surface of the object, while the time interval of not less than the width of the pulse is set between two adjacent irradiations. Thus, the pulse laser beams do not applied for an overlapped time period, so that they do not mutually interfere with each other or one another. At that time, the worked result is equivalent to that obtained by summing respective irradiation intensity distributions of the plural pulse laser beams. Furthermore, the worked shape can be flexibly changed depending upon the kinds of selected laser beams by the working method in which working is effected by selecting and using some of the plural split pulse laser beams.
The following are preferred embodiment of the laser working method according to the present invention. Any combination of the following embodiments should be deemed to be preferred embodiments of the laser working method according to the present invention, unless contrary to the present invention.    (1) The intensity of at least one of the plural laser beams is adjustable.
By this, the object can be worked in a three-dimensional shape, while relative intensities of the plural pulse laser beams are changed.    (2) The laser beam is split by using a diffraction optical element.
When the laser beam is split by using the diffractive optical element, the number and the intensity distribution of the split laser beams can be changed in various manners depending upon the design of the diffractive optical element. Further, when the laser beam is split with the diffractive optical element, the ultrashort pulse laser beams can be irradiated upon the same location, while a very short time interval is being ensured among the irradiated beams.    (3) A polarization beam splitter is used as the splitting optical element.
By this construction, the laser beam can be split into two laser beams by passing the pulse laser beam through the polarization beam splitter. At that time, the relative intensities of the two split laser beams can be varied by adjusting the polarization direction of the incident laser beam. As compared with a method in which the laser beam is to be split by a half mirror, loss of the beam can be almost completely diminished by using the polarization beam splitter in case that the laser beam is split into two laser beams and they are coupled together.    (4) The laser beam is split into plural laser beams and the split laser beams are subjected to time-delaying by using a birefringent material as a splitting/delaying optical system.
When a pulse laser beam is inputted into the birefringent material such as calcite, an optical path changes depending upon a polarization direction. The laser beam can be split by utilizing this phenomenon. When the laser beam is split by this method, time delay occurs between two split laser beams. The delayed time can be varied by the thickness of the birefringent material.    (5) The pulse laser beam is split into plural laser beams spatially.
The laser beam ordinarily has a spatially non-uniform intensity distribution, which causes poor worked shapes. In order to solve the spatially non-uniform intensity distribution of the laser beam, a beam homogenizer is usually used to split the laser beam spatially and gather the split laser beams one upon another at the working surface. However, mutually interfering laser beams cause interference at a location where the beams are interposed together. As a result, a non-uniform intensity distribution is formed by the interference. According to the working method of the present invention where the plural pulse beams obtained by splitting the laser beam three-dimensionally are irradiated upon the working surface while time delays are given thereto, the above interference does not occur, so that the object can be worked with the laser beams having a three-dimensionally uniform intensity distribution.    (6) The worked shape is controlled by projecting at least one of the plural pulse laser beams in a reduced size through a mask.
By this, the worked shape can be controlled. Particularly, since the shapes of plural mask patterns can be worked so that the worked shapes may be overlapped at the working surface, fine working can be easily performed. Further, working can be performed at high flexibility in that a slightly different worked shape can be realized by changing any one of the plural masks, for example.
Further, when a stage for moving the mask is attached or when an element, such as a liquid crystal, which can control the spatial intensity distribution of the laser beam, the worked shape can be dynamically adjusted.    (7) The worked shape is controlled by modulating the intensity of at least one of the plural pulse laser beams by a spatial intensity modulator.
It is known that light of which intensity is spatially modulated (amplitude modulated) forms a Fourier transformed image of the modulated image at a light-condensed point. In this embodiment, the worked shape can be controlled by this principle. When the spatially intensity modulator such as a liquid crystal is used, the worked shape can be dynamically adjusted.    (8) The worked shape is controlled by modulating the spatial phase of at least one of the plural pulse laser beams.
Since there is an optical transparent portion in the method for obtaining the Fourier transformed image by modulating the amplitude, a energy loss occurs for the incident light. On the other hand, almost the entire incident light can be utilized for working, so that working can be performed at a high energy efficiency. The worked shape can be dynamically controlled by using the spatial phase modulator employing a liquid crystal.    (9) A material exhibiting transparency for the laser beams and a different refractory factor to air is used as a delaying optical system.
This material can finely modulate the delayed time simply and inexpensively.    (10) An electro-optic element is used as the delaying optical system.
According to this embodiment, the delayed time increases in proportion to the intensity of current by utilizing the primary electro-optic phenomenon, so that the delayed time can be finely adjusted with good reproducibility.    (11) A material exhibiting transparency to the laser beam is used as the object to be worked.
This embodiment can effectively cope with the production of high-need products such as transparent type optical elements by working the transparent material. Particularly when a step or curved surface exists at the surface of the transparent material, a problem that the laser beam is partially concentrated inside the transparent material to cause an internal defect can be prevented.    (12) A thin film or a laminated film unit is used as the object to be worked.
According to this embodiment, when the thin film is used as the object to be worked, the film or film unit can be uniformly worked in a depth direction irrespective of the intensity. Further, the worked shape can be controlled in a nanometer order in a depth direction by using the laminated film unit.
The laser-working apparatus according to the present invention comprises:
a splitter for splitting a single pulse laser beam having a time width of not more than 500 picoseconds into plural pulse laser beams;
a time-delayer for subjecting said plural pulse laser beams to time delaying;
a beam shape-rectifying optical system for adjusting a working surface-irradiating shape, an intensity distribution and a working surface-irradiating position of each of the plural pulse laser beams; and
a beam-propagating optical system, wherein a target object to be irradiated is worked with the plural laser beams, while the respective working surface-irradiating shapes of said plural laser beams are partially or entirely spatially overlapped with each other or one another, and a time period during which each of the pulse laser beams is irradiateirradiatedd on a working surface of the object is set to a time interval of from a width of a pulse to nanosecond.
The present invention is also directed to an optical element produced by the laser working method or the laser working apparatus according to the present invention.